Group 10 - GM V-6 Engine Gate5

Contents

Delivery - Gate 5

Finalization Of Deliverables

System Design Revisions

Variable Valve Timing:

Figure 24

Figure 24 sourced from the Toyota Website shows the mechanical connections that alter the angle of the cam lobes.

This engine currently uses a fixed camshaft and therefore fixed valve timing. Variable valve timing allows the engine to adjust the valve timing depending on the engine speed. To fully understand why this is important one must first realize that the valves do not open and close at Top Dead Center (TDC) and Bottom Dead Center (BDC). It is best to break the process into its individual steps. During the intake stroke, air is sucked into the cylinder because of a partial vacuum created by the pistons movement. The incoming air has a mass and velocity so therefore it has momentum. This momentum causes the air to continue rushing into the cylinder even after BDC. The valve is held open for a certain time to allow this air to enter. If the valve is open too long reversion occurs which is when the air begins to escape through the intake valve. The valve timing should be adjusted so the valves closes right when this reversion begins to occur. For high RPM the valve is open longer and shorter for lower RPM. This allows the maximum amount of air to enter the cylinder for each engine speed. After the intake stroke the compression stroke begins. During this process both valves remain closed so the air/fuel mixture can be compressed for ignition. Once the piston reaches TDC the spark fires and ignites the fuel/air mixture sending the piston back down in the power stroke. Just before the piston reaches BDC on the power stroke the exhaust valve opens. This allows some of the exhaust gas to begin to release before the piston starts the exhaust stroke. This decreases the back pressure on the piston, which can rob horsepower from the engine. Properly adjusting this timing will minimize the internal power losses of the engine. Next, during the exhaust stroke, the exhaust valve is held open for a few degrees past TDC for the same reason as before. The exhausting air has momentum and continues to leave the cylinder. The goal is to get as much air out as possible so fresh air can fill the cylinder and variable valve timing can maximize this. Lastly, there is a period of overlap when both valves are open. Just before the piston reaches TDC on the exhaust stroke the intake valve opens. Air doesn't escape through the intake however, because the exhaust manifold creates a low pressure wave that pulls the air out of the cylinder faster than normal without letting it escape through the intake valve. Variable valve timing can adjust this overlap time to match the engine speed as well.

According to fueleconomy.gov, the average fuel savings is 5% and a total savings of $1,300. There is also the benefit of decreasing harmful pollutants produced by the engine. The appeal to a consumer base is that they will be able to obtain more efficient and environmentally friendly power from the same engine. Currently, there are several ways to implement VVT in engines, involving mechanical and hydraulic methods. The cost difference between an engine with VVT and one without considering the lifetime of the engine is nominal.

Figure 25 This piston shows the machined surface to increase internal volume of the cylinder

High compression pistons would be an possible design revision for the Vortec V6. They maximize the compression ratio within the cylinder which increases the thermal efficiency of the motor. High compression pistons have a dent forming out the top of the piston similar to a dome shape which decreases the volume of the area above the piston. By decreasing the volume, the pressure within the piston is increased. If the pressure gets too great, the fuel can auto ignite. The auto ignition throws off the timing of the system because the fuel ignites before the power stroke is supposed to occur. This occurrence is why there is a maximum compression ratio. Along with the pressure increasing from the high compression pistons, comes the necessity for reinforcement of the engine block. The new pressure could be higher than the engine block was originally designed to withstand. The block can be physically designed different to handle the higher compression ratio, or it can be made of a different, stronger material, such as aluminum. Other changes to the engine would include changes to emissions control systems, then engine computer (ECU). The price of all the components and the chance of ruining the engine make this option very risky and an option viable for only consumers who are proficient at tuning an engine. After the tuning to increase compression ratio is completed, a higher octane fuel would have to be used at all times to prevent detonation (auto-ignition of the fuel before the spark plugs can ignite it). The process would increase harmful emissions to the atmosphere and reduce the fuel efficiency. If a consumer would be willing to take the risk and disadvantages of harming the environment and paying more to run the engine, they would enjoy more power output of the engine. The reason engines are not tuned accordingly from the manufacturer is that the process can reduce the reliability of the engine. Therefore this option would never be reasonable revision for an engine manufacturer, but a possible revision for a tinkering consumer.

Use Overhead Cams:

The engine now has one camshaft located between the cylinder banks, which requires a series of lifters, lifter covers, and push rods to move the rockers which open and close the valves. Changing to an overhead cam system would eliminate these parts. This would move the camshaft from its location to the top of the cylinder head. The overhead camshaft would now directly push the rockers instead of going through the lifters and push rods. This would decreases the amount of moving parts and also decrease friction and power loss generated within the engine due to less reciprocating mass of the camshaft. This revision would require some other major changes in the engine. The timing system would need to be changed to connect both new camshafts. The rockers would also have to be redesigned to work with the cams instead of the push rods, but could also be eliminated with the right design. This would also decrease wear on the engine by decreasing the load on each camshaft and eliminating points of mechanical contact. From a manufacturers perspective, they could market a more advanced reliable product that uses similar if not fewer resources and manufacturing processes with a higher torque output at a comparable price per engine. The cost for redesigning the engine would be significant. That cost could be offset if the manufacturer raised the price of the engine as a more advanced product compared to competitors. The simpler engine design would also appeal to consumers as an engine that would have lower service costs in its lifetime.

This is an animation of an engine cylinder with a Single Overhead Cam, controlling the intake and exhaust timing for the values of a single side of cylinders. Note there are no pushrods.

Component Design Revisions

Stamped Steel Oil Pan

Stamped steel is stronger and more durable and resists impacts better than the engines current cast aluminum pan. Cast parts can crack when struck and this could cause a catastrophic failure of the engine, from loss of oil. The oil pan is located on the bottom of the engine and has a high risk of being hit by debris. Strengthening this part will make the engine more durable. While steel can rust, it can be coated with rust inhibitors to help prevent this. A steel pan would be more expensive but it would make a more durable engine.

Steel Oil Pan

Cast Aluminum Block

Aluminum is much lighter than cast iron with a density of about 2.7 g/cm^3 compared to cast iron at a density of about 7.85 g/cm^3. This change can greatly decrease the overall weight, allowing for a greater power to weight ratio. The downside of cast aluminum is that it is more expensive than cast iron. For example, if you were to buy an aftermarket aluminum engine block would cost about $4000 for a V8 while a cast iron block would be about $600 according to JEGS High Performance.

Aluminum Engine Block

Assymetric Water Pump Mount

This revision doesn't affect performance or efficiency but it does make maintenance easier. Through our experience we noticed that the water pump could be installed upside down without any major differences. Besides plumbing there are no geometrical differences between the pump in it's correct position and inverted. This could easily be fixed by making the mounts angled, this way the holes wouldn't line up when inverted.

Final Assessment

This project has revealed several possible design revisions for the GM Vortec V6. While they all add cost to the manufacturing process, they also make the engine better and more valuable to a customer. They are also very dependent on the environment. The engine would not need variable valve timing if it was used in a machine that ran at a constant speed, like a hydrostatic fork lift. Overhead cams and high compression pistons wouldn't be needed if the engine was only used in a small car. The engine wouldn't be as efficient as it could be but the price would be lower. The engine could keep its aluminum oil pan and cast iron engine block, unless it was used for off-roading or racing, where weight and power is important. Changing to an aluminum block would increase the power to weight ratio, adding the steel oil pan would add weight but also increase durability. This would be most helpful in a off-road setting.

Reassembly Details

Unfortunately, the engine was never in a fully working state. It was missing various fasteners and other major components such as a battery, alternator, radiator, spark plugs, and ignition system. The engine was merely a non-working model with almost all of the major components and served as a great learning tool.

The assembly process is essentially the exact opposite of disassembly. The main difference between reassembly and disassembly is that the orientation of certain parts needs to be taken into account for reassembly. The pistons and there respective connecting devices need to be properly aligned to the front of the engine and in there own specific cylinder to prevent excessive wear.

An additional recommendation would be to use the same type of bolts throughout the engine. There was a mix of metric and standard bolts used and it made dissection overly time consuming. There are only a few points on the engine where metric bolts wouldn't work. The sleeves that hold the cam and balancing shafts in the engine are torx bolts. These bolts can be used similarly to those for phillips or flat head bolts. The torx head can withstand higher torque than and is more reliable in automotive engines.